In land seismic surveys, a source induces seismic waves at or near the surface of the earth. These waves propagate through the earth and reflections from different layers within the earth can be detected by sensors, or geophones, at the earth's surface. The seismic source vibrations applied to the earth's surface also generate a so-called surface wave or ground roll which propagates through the shallow layers of the earth. At the geophones, the time of incidence of the low frequency, low speed ground roll typically coincides with the incidence of reflections from the deep layers of interest in the seismic survey. The simultaneous presence of the ground roll with the reflected signals makes it difficult to make full use of the seismic data as the ground roll often masks the reflected waves.
Several methods are known for attenuating ground roll interference and thus reducing its effect on the seismic signal of interest. Typically, geophones are not used individually, but rather are connected in sub-arrays, or groups, which are hard-wired or summed together. This is a form of data-independent beamforming. Attempts have also been made to apply adaptive signal processing for the suppression of ground roll in seismic surveys.
U.S. Pat. No. 4,556,962 attempts to attenuate the ground roll from a surface seismic source by placing a sensor close to the source to detect the interfering noise. The interfering noise is scaled, delayed and summed with signals from a more distant geophone array and then cross-correlated with the original vibrational source. This Patent also suggests that an adaptive filter be used so as to modify the delayed signal to correspond more closely to that detected by the more distant geophone array. However, the ground roll measured close to the source may be substantially different from that received by the geophone array, and the adaptive filter may not be able to deal with this.
In U.S. Pat. No. 4,890,264 a method for suppressing non-uniformly distributed noise generated by surface wave propagation, wind, and machinery is described. Horizontal geophones for detecting surface waves are used with conventional vertically orientated geophones for detecting seismic energy. The outputs of the surface wave detectors are used in conjunction with an adaptive filter to cancel the effects of the surface wave interference. This method for the suppression of ground roll is inherently a multicomponent method. Some seismic wave energy also gets detected by the horizontally sensitive geophones, and this may cause signal cancellation.
In UK Patent Application GB-A-2273358 the use of linearly constrained adaptive beamforming and adaptive interference cancelling beamforming for ground roll suppression was proposed. This method filters signals measured by an array of geophones and sums them in such a way as to preserve signals incident from a preferred direction while suppressing interference incident from other directions. The filtering is performed using a continuously adaptive method with the moveout differential between the seismic reflections and the ground roll being used to form primary and reference channels. The suggested application is in drilling when using a drill as a seismic source, where the ground roll is effectively stationary due to the slow travel of the drill bit and each source receiver position produces a lot of data. This ensures that the stochastic gradient type of algorithms used in the adaptive filters of this method are able to converge. However, in surface seismic experiments the ground roll present is often non-stationary and inhomogeneous and the stochastic gradient type of algorithms may be too slow to converge within the signal envelope.
U.S. Pat. No. 5,237,538 proposes a method for removing coherent noise from seismic data. This method firstly identifies the moveout characteristics of the noise, defines and extracts a space-time gate containing the noise, and removes the moveout to flatten the noise train. Amplitudes and time variations are then removed from the gate. The coherent noise is estimated using a beamsteer operator (conventional stacking in this case) or by f-x filtering in the Fourier tranform domain. The filtered noise estimate is subtracted from the data trace containing the signal-plus-noise using a short three to five point single filter. Inverse amplitude scalars are applied to undo the effect of earlier amplitude equalisation. The signal is then moveout restored into the original seismic record. This method has some particular shortcomings for application for ground roll attenuation. First of all, especially for shorter arrays, the signal always leaks into the ground roll estimate. In fact, there is always a component of the signal present at the reference channel which is colocated in time with the signal in the primary channel. On the other hand, when the arrays are allowed to be longer, the dispersion present in the ground roll make it difficult to achieve effective beamsteering.